1. Field of the Invention
The invention relates to a method and a circuit for generating a spot position indication signal in a cathode ray tube display arrangement, in which displays are realized by line-by-line scanning of a display screen of a cathode ray tube by an electron beam spot, the cathode ray tube display arrangement comprising a deflection circuit for generating a deflection current to deflect the electron beam spot in a first direction, wherein the deflection current has a shape for obtaining an approximately linear scan on the display screen in the first direction during a scan period.
The invention also relates to a cathode ray display apparatus comprising such a circuit for generating a spot position indication signal.
Such a spot position indication signal may be used for generating position dependent waveforms for correcting deflection errors of the cathode ray tube, such as convergence errors or east-west distortions, or for use as a dynamic focusing waveform, or as a waveform influencing the brightness of a displayed picture to compensate for brightness non-uniformity on the cathode ray tube.
2. Description of the Related Art
It is known to obtain a line position indication signal which indicates the vertical sport position on a raster scanned display screen for every line by using an analog-to digital converter (further referred to as ADC). The ADC measures a value of tile vertical deflection current at a moment a line occurs. The vertical position of the spot, and thus of a line on the display screen is determined by the value of the vertical deflection current in this line. Hence, the ADC supplies a line position indication signal that is a measure of the vertical position of the spot on the screen. If a repetition frequency, or an amplitude, of the vertical deflection current changes, still the ADC supplies the actual vertical spot position at the moment a line occurs as the vertical spot position is still determined by the vertical deflection current. For example, in convergence circuits using the output signal of the ADC for calculating convergence waveforms, the ADC must have a resolution of approximately 13 bits for display systems displaying about 600 lines. A striping would become visible on the screen if a lower resolution would be used. This striping is a brightness modulation caused by different distances between adjacent lines due to inaccuracy in the position of the lines. Such a high resolution ADC is expensive.
It is an object of the invention to provide a simple and cheap spot position indication signal.
To this end a first aspect of the invention provides a method for generating a spot position indication signal characterized in that the method comprises the steps of generating, in a certain scan period, a position information being related to the deflection current; measuring a first moment in time at which the position information has a first value; measuring a second moment in time at which the position information has a second value; calculating, in a later scan period, the spot position indication signal as a linear function in time whereby, in a steady state situation, the spot position indication signal has predetermined position indication values at corresponding first and the second moments in time in this later scan period, respectively.
A second aspect of the invention provides a circuit for generating a spot position indication signal the circuit receiving a deflection current for scanning an electron beam spot in a first direction along a display screen of a cathode ray tube line by line, wherein the deflection current has a shape for obtaining an approximately linear scan on the display screen in said first direction, characterized in that the circuit comprises means for generating, in a certain scan period, a position information being related to the deflection current; means for measuring a first moment in time at which the position information has a first value; means for measuring a second moment in time at which the position information has a second value; means for calculating, in a later scan period, the spot position indication signal as a linear function in time whereby, in a steady state situation, the spot position indication signal has predetermined position indication values at corresponding first and the second moments in time in this later scan period, respectively.
A third aspect of the invention provides a cathode ray tube display apparatus comprising a circuit for generating a spot position indication signal characterized in that the circuit for generating a spot position indication signal comprises means for generating, in a certain scan period, a position information being related to the deflection current; means for measuring a first moment in time at which the position information has a first value; means for measuring a second moment in time at which the position information has a second value; means for calculating, in a later scan period, the spot position indication signal as a linear function in time whereby, in a steady state situation, the spot position indication signal has predetermined position indication values at corresponding first and the second moments in time in this later scan period, respectively.
The invention offers a very simple and inexpensive method for generating a position indication signal representing the spot position on the screen of a cathode ray tube.
It is important to know the actual position of the spot if a waveform has to be generated which should be a function of the position on the screen. This means that at a certain position on the screen, a certain associated value of the waveform must occur. Such a position dependent waveform may be used to correct for position dependent geometry errors, such as east-west, north-south or convergence distortions, or to correct for brightness non-uniformity across the screen, or for obtaining a position dependent scan velocity modulation.
It is common practice to generate the position dependent waveform as a function of a variable depending on the time or a line number. The position dependent waveform which is generated in this way depends on the amplitude and the frequency of the horizontal or vertical deflection. For example, suppose, that the cathode ray tube display arrangement displays a PAL-picture with 625 lines which has a vertical amplitude suitable to scan the screen along the whole height. A position dependent waveform depending on time is generated to obtain a certain correction, for example, an east-west correction. This waveform has a suitable shape and extends, during a vertical scan period, over the whole vertical height of the screen. If, then, the amplitude of the vertical scan is decreased to scan only a part of the height of the screen, the generated position dependent waveform will have the same suitable shape extending daring the same vertical scan period, but now across only part of the height of the screen. As such, the values of the position dependent waveform occur at a wrong position, as now the same correction is performed on a smaller part of the screen. At different vertical frequencies for example: 50 Hz PAL, 60 Hz NTSC, and 45 Hz free running), a different total number of lines appears (as the line frequency does not change significantly) and thus, a different position dependent waveform is generated as the lines at different vertical frequencies appear at different positions on the screen.
Thus, the position address should be related to the vertical spot position at a moment a line in the picture occurs, to obtain a position dependent waveform independent of the frequency and amplitude of the vertical deflection. A same reasoning holds for a position waveform depending on the horizontal position on the screen.
The invention provides a position information signal (further referred to as address) generator based on the insight that the spot position on the screen is a linear function in time. The spot position is a linear function in time if the deflection current has a shape for obtaining a linear scan on the picture tube screen. Further, use is made of the fact that a certain deflection current corresponds to a certain position on the picture tube screen. As, in case of a linear scan, the spot position on the screen is a linear function of time, the address generator should generate an address which is a linear function in time. The address generator supplies an address which represents the spot position on the screen if the linear function in time representing the address is coupled to the linear function in time representing the spot position. Thus, the address generator supplies an address which is related to the spot position if two predetermined (desired) address values occur at two selected levels of the deflection current to which belong two positions on the picture tube screen. The actual value of an address at a certain moment depends on the choice of the predetermined address values which have to occur at the selected positions on the screen. Therefore, the invention determines, in one scan period, a first and a second moment in time at which a first and a second selected level of the deflection current is reached, respectively. The address which is generated as a linear function is related to the spot position if at the first and second moments in time, in a steady state, the address has the predetermined address values (position indication values), respectively.
An embodiment of a method for generating a spot position indication signal (or address) according to the invention, measures the two moments in time at which the two selected levels of the deflection current occur in a simple way. The count value occurring at the moment that a corresponding one of the selected levels is reached is stored.
In another embodiment of the method according to the invention, the address is written as a linear function in time which comprises an initial position indication value (further referred to as initial value) added to an incremental value multiplied with the time. The initial value and the incremental value used in a certain scan period are determined from two linear equations. The two linear equations are obtained by substituting, in the linear function representing the address, the two moments or the two mean values of these moments on which the deflection current reaches the two selected values in the preceding scan period, and the above mentioned predetermined address values at these moments, respectively. In this way, the address in the certain scan period is generated based on the initial value and the incremental value obtained in an easy way from the two linear equations.
In another embodiment of the method according to the invention again, the address is written as a linear function in time which comprises the initial value added to the incremental value multiplied with the time. Now, the initial value and the incremental value used in a certain scan period are determined in a closed loop manner. Therefore, the first and second values of the address occurring at the first or second moment, are determined. These first and second addresses are compared with the predetermined address values, respectively. The initial value and the incremental value for use in the certain scan period are determined from the difference between these first and second addresses as occurring in a preceding scan period and the predetermined address values. The determination of the initial value and the incremental value can be performed in numerous ways depending on the requirements that the method for generating an address has to fulfill. An advantageous embodiment is described in the description of the Figures.
Another embodiment of the method, according to the invention, generates the address in a certain line by adding the incremental value to the address of the preceding line. In this way the multiplication of the incremental value with the time is replaced by a simple addition.